1. Field of the Invention
The present invention relates to a two point modulator using a voltage control oscillator and a calibration processing method, and more particularly, to a two-point modulator calibrating a gain and nonlinearity of a voltage control oscillator, and a calibration processing method performed by the two-point modulator.
2. Description of the Background Art
As generally known, a voltage control oscillator (hereinafter, referred to as VCO) is widely used, as a device for generating a local oscillation signal, for a modulator of a radio communication apparatus. By using the VCO, a frequency modulated signal or a phase-modulated signal can be generated. Moreover, by inputting a modulated signal having a constant envelope, generated by the VCO, to a power amplifier, and controlling a supply voltage of the power amplifier, a modulated signal having a modulated component also on its amplitude can be generated (phase shift keying; PSK, code division multiple access; CDMA, orthogonal frequency division multiplexing; OFDM, etc.). In recent years, there arises a need for enabling an oscillation frequency of the VCO to be adjusted over a wide range of frequencies, in order to adapt the modulator to a communication system using a plurality of different frequency bands.
With that, in order to realize a modulator having a wide band, there is suggested a modulation method called two-point modulation. FIG. 13 shows an example of a configuration of a modulator using a conventional two-point modulation method.
As shown in FIG. 13, the conventional two-point modulator 501 includes an operation section 521, a frequency error calculation section 522, a loop filter 523, a adding section 525, a VCO 526, a frequency detection section 527, and a buffer 528.
A modulated signal is converted to a signal corresponding to a desired frequency channel by the operation section 521, and outputted as a low-pass response signal via a frequency error calculation section 522 and a loop filter 523. Meanwhile, a modulated signal is adjusted by the buffer 528 so as to be a desired signal, and then outputted as a high-pass response signal. The adding section 525 adds the high-pass response signal to the low-pass response signal, and inputs the resultant signal to the VCO 526. A signal outputted from the VCO 526 is fed back and inputted to the frequency error calculation section 522 via the frequency detection section 527. The frequency error calculation section 522 detects and outputs a frequency error between the modulated signal outputted from the operation section 521 and the signal outputted from the frequency detection section 527. This feedback processing stabilizes a frequency of the signal outputted from the VCO 526.
Thus, by using two-point modulation method, modulation characteristics including in a combined manner a frequency gain as a low-pass response via a feedback circuit and a frequency gain as a high-pass response via a feedforward circuit can be obtained, whereby a modulator having a wide band can be realized (FIG. 14).
However, even by using the above two-point modulation method, there remains a problem that, if the VCO 526 is a VCO exhibiting nonlinearity, a frequency band in which linear modulation can be performed is narrow, distortion occurs on the output, and, as a result, wideband frequency characteristics cannot be obtained (FIG. 15). Therefore, there arises a need for calibrating a gain and/or nonlinearity of the VCO 526.
U.S. Pat. No. 7,061,341 (Patent Document 1) discloses an invention for solving the above problem. FIG. 16 shows an example of a configuration of a conventional direct modulator 511 disclosed in Patent Document 1.
As shown in FIG. 16, the conventional direct modulator 511 includes a PLL circuit having a VCO 1506, a divide-by-N frequency divider (N counter) 1508, a phase comparator, a charge pump (CP), and an RC connection filter. A phase signal corresponding to a desired channel is converted to a digital modulated signal by ΔΣ modulator, and the digital modulated signal is supplied to the divide-by-N frequency divider 1508 and the phase comparator. A step signal ΔfPM is converted to an analog signal by a D/A converter 1510, and is inputted to an auxiliary terminal 1504 of a VCO 1506 via a low-pass filter (hereinafter, referred to as LPF) 1512.
In the above configuration, the PLL circuit is operated in a closed loop state. First, a desired channel frequency fc is inputted and the VCO 1506 is locked up at a division ratio N. A lockup voltage Vctrl used upon the lockup is held [fREF=fC/N]. Next, the step signal ΔfPM is inputted and the division ratio of the divide-by-N frequency divider 1508 is shifted by ΔN. Moreover, the step signal ΔfPM is adjusted such that the lockup voltage Vctrl used at this time is the same as the lockup voltage initially used [fREF=(fC+ΔfPM)/(N+ΔN)]. By performing the above processing for a plurality of calibration points, the conventional direct modulator 511 calibrates a gain or nonlinearity of the VCO 1506.
However, in the conventional direct modulator 511 disclosed in Patent Document 1, since the PLL circuit is operated in a closed loop state, it takes a long time to perform calibration processing on the VCO 1506. Therefore, in a communication system in which there is a restriction on lockup time due to the specification thereof, it can occur that the lockup is not completed in time. If calibration processing is started in a state where the lockup is not sufficiently performed (is not completed), frequency offset occurs.
In addition, during calibration processing, there is also a problem that the lockup voltage varies by leakage in a filter or a parasitic capacitance included in the apparatus, and thereby frequency drift occurs.